In steel, petroleum, chemical, cement, paper and pulp, ceramic engineering, biomass and other industries, a once through steam boiler or the like is used, which generates steam by use of heat of combustion of a fossil fuel such as oil, gas or the like in order to supply a heat source for a heating step such as a drying step, a sterilization step or the like. Recently, energy savings and carbon dioxide emissions reduction are serious problems in industries. It is considered that a heating method using a heat pump reduces carbon dioxide emissions as compared with a conventional method of combusting a fossil fuel to supply heat.
A heat pump is generally used for cooling or heating in an air conditioning device or a hot water supply device.
Chlorofluorocarbon (CFC) or hydrochlorofluorocarbon (HCFC) each containing fluorine and chlorine are conventionally used as a coolant of a heat pump cycle, but use thereof has been stopped step by step from the viewpoint of ozone layer protection. Today, a natural coolant such as ammonia, carbon dioxide or the like and hydrofluorocarbon (HFC), which does not contain a chlorine atom in a molecule, are mainly used.
A known heat pump for hot water supply uses carbon dioxide as a coolant. For example, PCT Japanese National Phase Laid-Open Patent Publication No. Hei 3-503206 discloses a heat pump for hot water supply that adopts a transcritical cycle using carbon dioxide as a coolant. The critical temperature of carbon dioxide is 30.98° C., and the critical pressure thereof is 7.38 MPa.
A heat pump known as a hot water supply device or a steam generation device uses HFC as a coolant. For example, PCT Japanese National Phase Laid-Open Patent Publication No. 2013-525720 discloses a coolant for a high-temperature heat pump, which has a condensation temperature of 70° C. to 150° C. This coolant is a mixture of 1,1,1,3,3-pentafluoropropane (HFC-245fa) and 1,1,1,3,3-pentafluorobutane (HFC-365mfc), and has a critical temperature higher than 150° C.
A heat pump known as a superheated steam generation device uses 1,1,1,2-tetrafluoroethane (HFC-134a) as a coolant. For example, Japanese Laid-Open Patent Publication No. 2009-85540 discloses a turbo compressor that compresses HFC-134a into a supercritical state and a high-temperature heat pump that causes heat exchange between HFC-134a in the supercritical state and water to generate superheated steam having a temperature of 120° C. The critical temperature of HFC-134a is 101.06° C., and the critical pressure thereof is 4.06 MPa.
Transactions of JSME (The Japan Society of Mechanical Engineering) (B) Vol. 79, No. 799 pp. 423-430 discloses a turbo compressor that compresses HFC-134a into a supercritical state and a high-temperature heat pump that causes heat exchange between HFC-134a in the supercritical state and compressed water to generate pressurized hot water having a temperature of 130° C. In Transactions of JSME (The Japan Society of Mechanical Engineering) (B) Vol. 79, No. 799 pp. 423-430, HFC-134a, which is used as a single coolant, is compressed by the turbo compressor from 1.2 MPa at 70° C. (inlet of the compressor) to 5.0 MPa at 145° C. (outlet of the compressor). In order to suppress the increase in the pressure of the gas discharged from the compressor, an intercooler is provided, so that the degree of superheat of the coolant at the inlet of the compressor is 22° C. (with respect to the evaporation temperature of 48° C., the temperature of the coolant at the inlet of the compressor is 70° C.). FIG. 4 of Transactions of JSME (The Japan Society of Mechanical Engineering) (B) Vol. 79, No. 799 pp. 423-430 suggests that in the case where the intercooler is not provided in the above-described superheated steam generation device, the pressure of HFC-134a gas having a temperature of 145° C. at the outlet of the compressor exceeds 5.0 MPa.
However, it is concerned that HFC, which has a high global warming potential (GWP), significantly contributes to the warming. Therefore, hydrofluoroolefin (HFO), which is a fluorine-containing unsaturated compound, is proposed as a substitute coolant having a low global warming potential.
For example, PCT Japanese National Phase Laid-Open Patent Publication No. 2007-535611 discloses using, as a working medium of an air conditioning device for an automobile, a composition containing 2,3,3,3-tetrafluoropropene (HFO-1234yf) and polyalkyleneglycol (PAG) lubricant.
Japanese Laid-Open Patent Publication No. 2010-47754 discloses using, as a working fluid of a low-temperature freezer, a mixed compound of tetrafluoropropene such as 2,3,3,3-tetrafluoropropene (HFO-1234yf), 1,3,3,3-tetrafluoropropene (HFO-1234ze) or the like and difluoromethane.
Patent Literature 6 discloses using difluoromethane (R-32) or a mixed coolant containing R-32 and tetrafluoropropene (HFO-1234yf or HFO-1234ze) in order to replace a medium in a freezing cycle of increasing the temperature of water used as a second heating medium to 70° C.